The Problem
Each step in the process of fabrication of a semiconductor integrated circuit requires precision given the small size scale. Focusing, during projection lithography, of the image of a mask projected by a lens or other imaging device onto a photoresist layer located on top of a wafer, as with other steps, requires a high degree of accuracy. Although the surface of the photoresist layer is generally planar, enough significant variations in surface profile exist to create focusing problems.
Typically, the wafer has multiple locations at which fabrication of integrated circuits occurs. Present day technology and the unevenness of the photoresist layer on the wafer only allow exposure of one or two chip sites at a time, although many more integrated circuit locations exist on the wafer. Consequently, the wafer used in the process is normally moved to expose the photoresist layer at each location. The movement of the wafer during the fabrication process and the inherent unevenness of the photoresist surface necessitate the refocusing of the image of the mask on the photoresist surface with each move.